Optical fiber cables have been used for many years to transmit information at high rates and very long distances. The transmission media of the optical fiber cable are hair-thin optical fibers protected from external forces and elements by precisely designed and manufactured cable structures. There are several relatively common cable structure families currently being used to protect these hair-thin optical fibers. Such cable structure families include the loose tube, the slotted core and the central core structures.
Optical fibers are relatively delicate compared to previous types of communication media. Typically made of glass, the fibers are not ductile and can be broken or cracked, either of which can destroy or degrade the signal being transmitted. Therefore, optical fibers are housed in rugged cable structures to protect the fibers from mechanical damage caused by heavy loads, sharp impacts or gnawing rodents. The quality of a signal transmitted through an optical fiber is also sensitive to tensile stress. Accordingly, tensile stiffness members are included in each type of optical fiber cable structure to carry the majority of tensile loads.
Plastic jacketing materials are extruded as the final layer of fiber optic cables in a continuous, high-speed sheathing operation. This layer is applied directly over the layers of components used to carry tensile loads and to shield the optical fibers from the environment. Once added, the jacketing plastic couples to the components in contact with them. Several types of strength members are used in cables, both metallic and dielectric. Heretofore, strength systems used in optical cables have had two primary functions: 1) to carry tensile loads and, 2) to restrict the contraction and expansion of the jacketing plastic. Tensile loads are applied to the cable during installation, e.g., while being pulled into ground conduits, or during service life, e.g., while suspended between telephone poles. Environmental temperature variations during product life can cause the jacketing plastic to expand and contract applying tensile and axial compressive loads on the strength systems. However, a more significant instance of contraction by the jacketing plastic occurs during manufacture of the cable. Current jacketing materials, including high-density polyethylene and medium-density polyethylene, exhibit sufficient post-extrusion shrinkage during cooling as to apply compressive axial loads on any adjacent components e.g., strength members extending axially along the cable. To resist these compressive and the other tensile loads, rigid strength rods, made from steel, epoxy/glass, epoxy/aramid, etc., have been embedded in cable jackets.
In general, metallic components, including wires and armoring tapes, have excellent tensile properties and sufficiently high compressive stiffness so as not to strain significantly under the axial compressive load exerted by the cooling jacket. Alternately, dielectric materials, such as aramid and glass fibers, which provide excellent tensile characteristics, provide little axial compressive resistance in their simplest, and therefore lowest cost, forms, e.g., as yarns. Because these yarns cannot resist compressive loads, the shrinking jacket buckles these dielectric tensile members as it cools. Then, when the final cable is loaded in tension, the tensile yarns will not support any tensile load until the cable has been elongated to the point where the yarns are straightened. This process leaves the optical fibers vulnerable to tensile loading until the tensile yarns pick up the load. As a result, current dielectric strength members are generally constructed as rigid composite rods made of high modulus and high strength fibers, such as glass or aramids, embedded in a hard epoxy matrix. However, these rigid dielectric rods can be very costly, and they, along with the metal wires, make the cable less flexible because their added compressive stiffness leads to higher flexural stiffness.
In illustration of this prior art, U.S. Pat. No. 5,131,064 to Arroyo, et al., discloses a cable having strength rods and a lightning-protective sheath system comprising a thermal barrier, which are disposed between the core of the cable and its plastic jacket. The thermal barrier comprises a textile of glass yams that have been woven into a unit and then sandwiched between a pair of tapes together with a waterblocking material such as a superabsorbent powder. The glass yarns undulate in the longitudinal direction, not only because of their weaving pattern, but also because the tape follows the undulations of a corrugated metallic shield. Such undulations preclude the tape from receiving any portion of the load until the cable has already been elongated. And since the disclosed tape has a very low tensile strength, 420 Newtons per centimeter of width, the cable's tensile strength effectively comes from rigid strength rods that are embedded in the plastic jacket. However, these rods are less flexible than the woven tape, thereby reducing the flexibility of the entire cable. Further, if a pair of rods are used and are positioned diametrically opposite each other on either side of the core, they make the cable inflexible in all but one plane and much more difficult to handle and install.
Another example is U.S. Pat. No. 4,730,894 to Arroyo, which discloses an optical fiber cable that includes a plurality of equally spaced strength members disposed on a carrier tape and held in place by an adhesive. Once a plastic jacket is extruded onto the strength members, they are coupled to the jacket and provide tensile strength to the cable. However, if the strength members are flexible, i e., they have essentially no compressive strength or stiffness, then they will shrink in the longitudinal direction after the plastic jacket cools and will not be able to receive any portion of the tensile load until the jacket is elongated. This is undesirable because excessive cable elongation can cause the tensile load to be transferred to the optical fibers. On the other hand, as stated previously, increased compressive stiffness correlates to increased flexural stiffness and, therefore, decreased cable flexibility, which makes cables more difficult to handle and to install. However, to protect the valuable optical fibers, cable flexibility generally has been sacrificed in the prior art.
Yet another example of prior art, U.S. Pat. No. 5,838,864 to Patel, discloses a cable with a dielectric strength member system that attempts to maximize the flexibility of the cable by using a flexible woven strength tape to carry to majority of the tensile loads. To control post-extrusion shrinkage, two rigid epoxy-fiber rods are embedded in the jacket, diametrically opposite one another, on either side of the core. However, as these rods do not have to carry tensile load, their size is minimized and, therefore, the overall cost of the strength system is reduced. Further, the volume of jacket material required to encase the smaller strength rods is less than for larger rods, further reducing the cost of the overall cable sheath. Still, the strength system is more expensive and complex than is desirable because of the need for two types of strength systems.
Another approach to preventing jacket shrinkage and buckled strength members from making the optical fibers vulnerable to tensile loads is to control the length of the optical fibers relative to that of the sheath components. Fibers can be protected during the elongation of the sheath if they also are not straight when the cable starts to stretch. Accordingly, most fiber optic cables contain excess fiber length relative to the final length of the cable sheath. However, there is a limit as to the amount of excess fiber length that can reasonably be introduced into a cable core without making the cable diameter prohibitively large or inducing optical signal loss by introducing excessive bends or undulations along a length of fiber. Unfortunately, the contraction of standard cable jacket materials can buckle the simple dielectric strength members as much as 1.5%, while acceptable amounts of excess fiber length may be less than 0.5%.
Therefore, since excess fiber length alone cannot be used to compensate for jacket shrinkage, the common default solution to this problem is to use the rigid dielectric strength rods to resist jacket shrinkage. As stated previously, unlike the metallic strength members, these dielectric rods are very costly, even when compared to their base component yarns and filaments, and rigid enough to reduce cable flexibility.
Therefore, there appears to be a fundamental conflict in providing an easy-handling, cost-effective dielectric cable sheath and a dielectric sheath that adequately protects the optical fibers. Known designs for best protecting the optical fibers make the cable more expensive, stiff, and difficult to handle. Known designs for making the cable flexible either require that the glass fibers have more excess length than is desirable, or expose the glass fibers to tensile loading and possible breakage.
However, if the jacket material had minimal post-extrusion shrinkage, ie., less than the excess length of the optical fibers, the more costly dielectric strength components would not be required. Accordingly, what is sought is a jacket material with minimal post-extrusion shrinkage that still meets the demands of ruggedness and flexibility for use in outdoor communication cables.
Heretofore, some prior art cables have incorporated nucleating agents, e.g., inorganic materials, salts of aliphatic monobasic or dibasic acids, or alkali metal or aluminum salts of aromatic or alicyclic carboxylic acids, and filler materials, e.g., talc, glass fiber, and glass spheres, into buffer or core tubes to give the desired properties of high strength, low shrinkage, good flexibility, improved processibility and low cost. An example of such a prior art cable is described in U.S. Pat. No. 5,574,816, (the '816 patent) issued to Yang, et al. Because the fillers or nucleating agents of the '816 patent were only added to the core or buffer tubes, the cables of that design still required tensile stiffness members with sufficient compressive stiffness to resist jacket shrinkage. As mentioned hereinbefore, it would be desirable to eliminate the need for the costly dielectric strength members. Furthermore, the '816 patent only added the fillers to a polyethylene-polypropylene copolymer resin and does not address the use of fillers that can be added to other types of resin.
Thus, a heretofore unaddressed need exists in the industry to address the aforementioned deficiencies and inadequacies.